1. Field of the Invention
The present invention relates generally to the field of optical lithography and, more particularly, to a high-transmission attenuating phase shift mask (attenuating PSM).
2. Description of the Prior Art
In the circuit making processes, lithographic process has not only been a mandatory technique but also played an important role in limiting feature size. By lithographic process, a wafer producer can precisely and clearly transfer a circuit pattern onto a substrate. In a lithographic process, a designed pattern, such as a circuit pattern or a doping pattern, is created on one or several photo mask, then the pattern on the mask is transferred by light exposure, with a stepper and scanner, onto a substrate.
While the traditional technologies continue to advance at breakneck speed they are becoming quite costly and are no longer able to provide the resolution and depth of focus (DOF) with an acceptable process window on their own. This is a direct result of the Sub-Wavelength environment. Fortunately there is an additional knob the industry can adjust to improve the overall system performance. The field of low k1 lithography includes such techniques as optical proximity correction (OPC), phase shift masks (PSM), off-axis or modified illumination, spatial filters and high contrast resists. These techniques, collectively referred to as Resolution Enhancement Techniques (RETs), work in conjunction with the traditional techniques of decreasing wavelength and increasing NA to extract the highest level of performance possible from the advanced lithography systems.
Central to the resolution enhancement techniques are PSMs. The principle PSMs deployed in the industry include attenuated (usually used for contacts and metal layers) and alternating aperture (used for CD control for gates). The phase shift masks are the masks with additional phase shifter material layers selectively between the metal Cr line patterns. When the light passes through the phase shifter material layer of the phase shift masks, the phase of the electric field of the light is shifted exactly 180°, so the phase difference between the incident light and the shifted light is exactly half-wavelength and destructive interference is therefore formed on the wafer. The interference effect of the diffraction is resolved by the destructive interference and the resolution of the border of the metal lines is greatly enhanced.
At present, the attenuating PSM can be sub-classified into two types: low-transmittance type (T˜6%) and high-transmittance type (T>15%). As the transmittance of the attenuating PSM increases, problems arise when using these high-transmittance attenuating PSMs to create some specific patterns. Please refer to FIG. 1 and FIG. 2. FIG. 1 is a planar view illustrating a portion of a high-transmittance type (T=15%) attenuating PSM 1 and a dummy pad pattern 10 thereon. FIG. 2 is the after-exposure simulation result. As shown in FIG. 1, the dummy pad pattern 10 is situated on a quartz substrate 12 and is composed of a phase shifter. The dimension of the dummy pad pattern 10 is 800 nm×800 nm. According to the simulation result as shown in FIG. 2, the central portion of the photoresist corresponding to the dummy pad pattern 10 on the high-transmittance type attenuating PSM 1 collapses after exposure and development, thereby forming a recess defect 18 on the photoresist pattern.
FIG. 3 is the after-exposure simulation result when using a high-transmittance type (T=30%) attenuating PSM to form a dummy pad pattern that is the same as pattern 10 set forth in FIG. 1. As shown in FIG. 3, as the transmittance within the dummy pad pattern 10 increases to 30%, the central portion of the photoresist pattern is completely exposed to form a collapsed central aperture 28. The simulation result indicates that the pattern transfer of dummy pad pattern 10 when using the high-transmittance type (T>15%) attenuating PSM fails.
In light of the above, there is a strong need in this industry to provide an improved attenuating PSM to cope with the situation when using a high-transmittance type (T>15%) attenuating PSM to perform pattern transfer of some specific patterns, particularly those isolated pattern having relatively larger surface area and line pattern having large line width and large space.